Radio broadcasting in the United States began in November, 1920 when KDKA,
Pittsburgh was granted its call letters and began broadcasting
1. This development represented a challenge to the
phonograph, since free music and entertainment was available "off the air".
Phonograph recording companies sought to respond by improving the quality of
their recordings by adopting new technology.

A key innovator of this technology was Bell Laboratories. Bell
Laboratories was the research division of the US telephone monopoly,
American Telephone & Telegraph. Research had previously been done by
Western Electric, the manufacturing arm of AT&T, and also by Bell System
division until "...On January 1, 1925, AT&T established Bell Telephone
Laboratories, Inc., in New York City as a joint venture with Western
Electric. Gifford [President of AT&T] appointed Frank Jewett president
of the new company in charge of 4,000 workers..." 2.
Thereafter, a steady flow of ground-breaking technology was developed by
Bell Labs during the next 60 years.

Bell and Western Electric research had been working since at least 1914 on
technologies for long-distance telephone transmission to span the US.
This research also provided the components of what later became the Westrex
electrical recording system.

The first complonent was the condenser microphone, developed in 1916 by the
brilliant electrical engineer Edward Christopher "E.C." Wente (Ph.D. from
Yale in 1918). The condenser microphone became practical with the
development of the vacuum tube (valve), used to amplify the low-level signal
output of the condenser microphone
4.

Prior to the condenser microphone, since the earliest days of Bell's
telephone, the carbon microphone had been used as a transmitter (and
continues to be used today). The carbon microphone is basically a
generator of voltage, and requires no amplification, at least for telephone
transmission over distances up to several hundred miles (or kilometers).
However, it does not produce an electrical signal sufficient for
transcontinental telephone transmission. The condenser microphone
connected to an amplifier was found to produce strong, transcontinental
signals.

The condenser microphone design is innovative. It includes two
parallel plates, each having an electrical charge, The front plate moves
with the audio signal, while the rear plate is fixed. Sound waves moving
the front plate cause a slight change in the capacitance between the plates,
which allows a change in current flow in the electrical circuit connected to the
microphone transmitter. The amplifier circuit then amplifies this
low-level signal to a useable level 12.

This condenser microphone, although first developed for long-line telephone
transmission, when the US was beginning transcontinental telephone service,
had remarkable acoustic performance. In its initial versions with
thicker steel plates stretched to give stiffness (so as to raise its
resonant frequency above the audio range), it had nearly flat frequency
reproduction up to about 6,000 Hz. With improved, thinner plates of
aluminium, this reproduction was expanded up to 15,000 Hz, at a time when
the acoustic recording process did not record much above 2,400 Hz 9.

Prior to the Bell Laboratories research, a number of attempts had been made
to develop an electrical recording process that involved microphones and
electrical disk cutting. This was to replace the acoustic or
mechanical recording technique used since Edison, which involved no
electricity or amplification. An early example of electrical recording
was the work of Lionel Guest and Horace O. Merriman in Britain. They
had developed and demonstrated an electrical recording system using carbon
microphones connected to a moving coil recording head
15. Their system produced the earliest issued electrical
recording which has come down to us. This was the 1920 recording of
the burial of the Unknown Soldier in Westminster Abbey, London on November
11, 1920 14. Guest and Merriman placed 4 carbon microphones
inside Westminster Abbey 15, with their recording apparatus outside. The resulting
recording was issued by Columbia Graphophone Co. However, the sound
quality of the Guest and Merriman system was not satisfactory. British
recording companies, including Columbia Graphophone, after evaluation
decided not to adopt the Guest and Merriman system. They subsequently
went to the U.S., but with similar results, and their system was not further
developed.

The sound quality of the 1920 Westminster Abbey recording is sufficiently
poor that it is difficult to discern if only instruments are playing, or if
there is also singing. The result is significantly poorer than an
acoustic recording of the period, except perhaps that it would be difficult
for the acoustic horn to capture such a large group. You may hear and
judge the sound quality of this 1920 recording for yourself by clicking on
the link below. This reproduces the music Abide with Me from
this 1920 Westminster Abbey service.

In about 1920, two teams at Bell Laboratories were created to pursue
electrical recording. Joseph P. Maxfield and Henry C. Harrison, two of
the leading developers of modern electrical components at Bell Labs, were
charged with developing a phonograph recording system. E. C. Wente,
Donald MacKenzie and Irving Crandall (who had originally hired Wente in
1916) 19 were assigned to develop a sound system for films to be
shown in cinema theaters5. Both groups were overseen by
Dr. Harvey Fletcher
, who previously joined Bell research in 1916.

Dr. Harvey C. Fletcher was a leading research physicist born into a Mormon family
in Utah on September 11, 1884. Fletcher studied and worked at the
University of Chicago with Nobel Prize winner Robert A. Millikan. In 1911,
Fletcher received his Ph.D. in Physics from the University of Chicago summa
cum laude. After creating and heading the Physics Department at
Brigham Young University in Utah, Fletcher in 1916 became Director of
Research at Bell Laboratories, where he oversaw three decades of research
and improvement in sound, hearing, transmission, and reproduction.

Microphone design was a key part of the Bell Laboratories development of
electrical recording technology. Carbon microphones, using loose
carbon granules were employed from the earliest days of telephone
instruments until today. The carbon microphone alters the transmission of
electrical current from acoustic vibration. It requires no subsequent
electrical amplification, although the circuit to which it is connected needs
to have a voltage/current for transmission. In the 1920s, Bell
Labs developed improved designs of carbon microphones, including the so-called
"double button" carbon microphone separated the carbon powder from the
microphone diaphragm, reducing noise and improving clarity.

The famous Western Electric double-button carbon microphone in the Western
Electric 1B microphone housing circa 1925.

The double-button carbon microphone not only reduced noise and also greatly reduced
harmonic distortion. These improved carbon microphones and later the
condenser microphones described above were manufactured by Western Electric
(the manufacturing arm of the Bell system). The Western Electric
double-button carbon microphone in the 1B and 1C housing was widely used in
radio broadcasting. In broadcasting, the carbon microphone was
connected directly to the engineering control panel and then to the radio
transmitter. A radio studio from late 1924 with this microphone
arrangement can be seen in in the photograph below.

Art Gillham (with glasses) standing to left of Will Rogers in November 4,
1924 during the Eveready Hour, broadcast by WEAF New York. AT&T sold
WEAF in 1926 as part of a settlement with GE/RCA to exit broadcasting, and
GE and RCA committed to transmit all their network broadcasts via AT&T long
distance lines 18. (note the Western Electric carbon microphone in the 1B
housing on a stand)

Harrison and Maxfield made a number of developments which, combined
together, allowed the creation of an electrical disk sound recording system.
Already described is the condenser microphone. Another of these key
developments was the matched-impedance amplification system, having such a
condenser microphone, linked to a tube (valve) amplifier, with the amplified
output voltage driving a moving magnet (called also a "moving armature")
cutting head to scribe the sound into a wax master disk.

Bell Laboratories had early licensed the patents covering Lee de Forest's
Audion tube. Bell Laboratories discovered a number of wrong solutions which
de Forest had introduced - such as fillin his Audion glass tube with ionized gases.
Bell Labs determined that the gas was not as effective as a vacuum, and also improved
the internal construction. Bell Laboratories engineer Harold
Arnold had by late 1914 developed an improved triode vacuum tube, replacing gas
by a vacuum and redesigning the electrodes and filaments. This producing
an amplifier having low distortion and good linear amplification 20.
This newly perfected, efficient electrical amplifier Bell Laboratories delivered
to the Bell System to make trans-continental
telephone communication practical for the first time.

This amplifier was also an important part of the matched-impedance
electrical recording system which was developed by Bell Labs in the early
1920s. Performance was encouraging.
Initially, it had a a recording bandwidth from 50 Hertz to about 6,000
Hertz, beyond which its high frequency sensitivity declined11.

Frequency response of the electrical amplification and

condenser microphone of the Westrex system 11

This frequency reproduction range was noticeably superior to the highly
variable acoustic process, which (with the best equipment and with the best
recording engineers) could reproduce from about 250 Hz to about 2,400 Hz,
with rapid reproduction fall-off thereafter. This wider bandwidth
added another octave of sound reproduction, along with reduced harmonic
distortion and a generally more realistic sound image, including recording
of higher harmonics of the musicians. The dynamic range of the sound
recording and the background noise were both also somewhat improved.

At the same time, Edward C. Wente and his team had developed the "light
valve", a light-emitting vacuum tube that converted audio voltage into
variable levels of light to expose movie film. This light output
produced a variable density sound pattern on the film. Wente was
granted a patent on this technology in 1923, and it was later the basis for
the Western Electric "sound-on-film" process.

Wente's early condenser microphone designs continued to be developed into
continuously improved microphone transmitter designs. The Western
Electric Model 361 seems to have been available by late 1924 and the Model
394 transmitter by late 1926 (although these dates are my estimates based on
contemporary literature). According to tests, the Model 361 had had
several mid-range resonant frequencies, particularly "...a +7dB resonance at
2.9kHz..." 22. The later Model 394 microphone transmitter sought to
improve this and other characteristics. The Model 394 transmitter was
widely used in a number of Western Electric microphones for broadcasting,
movie and disk recording in different versions from about 1927 6
and into the 1930s.

The improvement of the condenser microphone and its use for 1925-1930
electrical recordings, was key in the quality of the Bell Labs system.
It should be added that there is disagreement among experts as to whether
the initial recordings made with the Western Electric electrical recording
system were made with carbon microphones or with condenser microphones.
However, it seems clear that by later 19269 or early 1927, condenser
microphones were definitely used in the Western Electric system. This
was important not only because of the condenser microphone's expanded
frequency range and lower harmonic distortion, but also because even
well-engineered carbon microphones produced a low level hissing or sizzling
background sound from the vibration of the carbon granules.

However, there were some inconveniences with early condenser microphones.
The condenser microphone transmitter required vacuum tube (valve) circuitry
to be located near the microphone, since the microphone transmitter required
a polarizing voltage to be applied to its condenser plates, and it also
required amplification. The output of the microphone transmitter was weak.
It also had a high impedance. If this output were sent down a long
electrical wire to the recording electronics, it would be degraded by noise.
Therefore, in condenser microphones used for recording from in this period,
the amplifier was placed in a box below the transmitter, or in another
nearby housing. In later Western Electric models, the transmitter was
sometimes attached to a tubular housing containing the amplifier circuits
and the circuits to provide voltage to the plates. Such a base was
initially large, as can be seen in the photograph below: the model 47A,
containing the 394 transmitter at the top, and the electronics in the
tubular base.

Western Electric 47A Condenser Microphone with 394 transmitter at top and a
single vacuum tube (valve) amplifier in base

The initial condenser microphones using the Model 361 transmitter were
replaced by improved condenser microphones such as the Western Electric 47A
having the Model 394 transmitter beginning in late 1926 and into 1927-1928
in both movie sound recording and phonograph disk recording
6.

Electrical recoding systems previous to and contemporaneous to the Bell
Laboratories development may be fairly described as being mostly developed by
empirical or 'trial-and-error' development. In contrast, Bell Laboratories
engineers, being fundamentally oriented to physics and electrical engineering
theory based their design on these scientific concepts. This reflected the
advanced electrical engineering eduction of these scientists; some of the few
working in such research in that era. In their
development of an electrical phonograph disk recording system, the Maxfield
and Harrison team based their system on the theory of a many-sectioned
electrical filter 5. Bell Laboratories had previously applied
filters to the improvement of
long-distance telephone transmission. The "theory of electrical
filters" was well-developed and mathematically well-characterized.
This allowed them to apply these known mathematical solutions to the
recording and reproduction problems they were attacking.

Using this approach, a mechanical equivalent of each electrical component of
such a filter was identified. For example, an electric filter terminated
in a resistor. Bell Labs engineers terminated their electrical recording
system in a rubber column in the disc cutting arm - functionally equivalent to
a resistor. This approach was used both for the
electrical recording system and for sound reproduction systems such as loud
speakers. In the case of the loud speaker, the air in an enclosed cabinet
could be considered to function like a capacitor in an electrical filter.
Since the mathematical solutions in the theory of electric filters had already
been solved, this accelerated a scientifically-based development of
the Bell electrical recording and reproduction systems.

In the new Bell Labs recording system, the microphone was connected to the
matched-impedance amplifier (described above), whose electrical output was
connected to and controlled the electrical disk cutting mechanism.

For the disk cutting mechanism, the Bell engineers developed a "moving magnet",
- also called a "moving armature" electromagnetic device. In this system,
the electrical output of the matched-impedance amplifier was fed to an
electro-magnet, causing the cutting stylus to move according to the changes in
the magnetic field of the electro-magnet. In this way, the music, in the
form of the varying electrical output of the amplifier, going to the cutting
stylus via the moving magnet would inscribe the music wave form into the wax
master. So, in this electrical system, the microphone and amplifier,
moving the stylus, replaced the old acoustic horn mechanically vibrating a sound
box which would move the stylus. Since the microphone and amplifier had a
broader frequency range and less harmonic distortion and less acoustic
resonances, the recording cut into the wax was dramatically better, as could
immediately be heard in the first electrical disks released by the Victor
Talking Machine Company. (See
1925 - The First Electrical Recording
to read about the world's first electrical recording of an orchestra.)

There were a number of limitations of this new electrical system which the early
recording engineer needed to overcome. If the electrical signal for the
music exceeded certain limits, it could cause the electromagnet-stylus assembly
to move too far and become stuck. To solve this, a small spring was added
to the cutting head to control stylus excursion.

Another difficulty was in the conversion by the Bell engineers of an electrical
component into an equivalent mechanical component. As described above, the
Bell Labs design was in part based on the "theory of electrical filters".
Electrical filter theory required that the final section of such a filter
terminate in a resistor. The mechanical equivalent of the electrical
resistor that the Bell engineers developed was a 9 inch long rubber line or rod.
This rubber line was added at the end of the system to provide mechanical
resistance or damping 5,10. The rubber rod was placed up the
inside of the length of the cutting arm. This is the source of the
description of the system by the engineers as being the "rubber line
recorder".

Without the damping resistance of the rubber line, the movement of the recording
stylus at high volume levels, particularly of low frequencies, would have been
excessive. The damping of the rubber line was one of the essential
developments that would make this Bell system linear in its frequency response
8.

However, this rubber tube also introduced difficulties. First, it required
careful design and repeated testing of the stiffness and compliance of the
rubber column. The coupling of components, the mass of the components, and
characteristics of the rubber each had to be varied and tested empirically to
discover a combination that gave the disk cutter an essentially flat frequency
response. In fact, in the lab, the Bell system was able to achieve a
remarkably flat response from 250 Hz to about 15,000 Hz 9, although
the total system in use commercially had an upper frequency response to about
6,000 Hz in the early years (progressively expanded each year thereafter).
It was also found later, from experience, that the recording engineer would need
to carefully pack and adjust the rubber line in the recording arm, prior to each
recording session to assure good linearity and response in the recorded sound.
In this way, the early Westrex electrical system, like the earlier acoustic
recording system. depended on the craft and resourcefulness of the recording
engineer to gain the best recording results from the system.

The microphone, plus the amplifier, plus the electromechanical disk cutting
mechanism combined together was the new Western Electric electrical recording
system, commercialized as the "Westrex" system.

Western Electric engineer George Groves cutting an electrical master.

Groves was assigned to Vitaphone, and later relocated to Hollywood

where he became one of the great sound engineers, winning 2 Oscars
9

By early 1924, the Western Electric system was ready for demonstration.

Western Electric's next task was to interest the phonograph recording
companies to license and adopt the Westrex system, to replace the acoustic
recording processes used for the last 50 years. The fascinating and
often surprising story of this is quest to implement electrical recording of
the phonograph disk can be seen by clicking on the link:
Licensing the Westrex Electrical Recording System to Victor and Columbia.

Note: Of course, the Western Electric electrical recording system, while
being the most successful early electrical phonograph recording system was
not the only, or even the first electrical system. Allan Sutton in his
fascinating and meticulously researched book:
"Recording the Twenties. The Evolution of the American Recording Industry,
1920-29"13, published in 2009 by Allan Sutton's
excellent
Mainspring Press describes the history of the development of the various
technologies leading to electrical recording. Well worth a purchase
and multiple readings.

20 page 334-348. Maxfield, J. P. and Harrison, H.
C. Methods of High Quality Recording and Reproducing of Music and
Speech Based on Telephone Research. Transaction of the American
Institute of Electrical Engineers. February 1926.

21 Frederick, H. A. The Development of the
Microphone. Journal of the Acoustical Society of America. July,
1931. New York, New York.